[RE-wrenches] sizing a sub-panel used tocombinemultipleinverteroutputs

Wed Mar 30 09:10:31 PDT 2011

Hmmm. That's not the way I understood it. I could be wrong.

690(B)(2): The sum of the ampere ratings of overcurrent devices in
circuits supplying power to a busbar or conductor shall not exceed 120
percent of the rating of the busbar or conductor. In systems with
panelboards connected in series, the rating of the first overcurrent
device directly connected to the output of a utility-interactive
inverter(s) shall be used in the calculations for all busbars and
conductors.

The way I read it, both the inverters and the feeder circuit are
supplying power to the conductor between the main and subpanels. The sum
of the OC protection is 100A + 80A = 180A per leg. Therefore, the
conductor must be rated for 180A / 1.2 = 150A. Maybe this changes if
there are no loads in the subpanel, but I don't think so. In many cases
we have a circuit for an energy monitoring device in the subpanel, so we
almost always have loads anyway. Another way to read it would be that
ONLY the inverter circuits shall be used for calculations, in which case
the conductors would need to be 100A / 1.2 = 83.3A. However, I don't
think that is the intent.

While on the subject, I always wondered about that Lennox "Solar A/C"
system that uses Enphase backfed through the A/C compressor circuit. It
seemed like the conductor would need to be upgraded.

Jason

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark
Frye
Sent: Wednesday, March 30, 2011 11:36 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used
tocombinemultipleinverteroutputs

Jason,

I think, going back to the diagram in the article, the feeder breaker is
rated at 80A, the feeder conductors are rated at 80A, and the subpanel
is rated at 80A, assuming that there are no loads connected in the
subpanel. Even if there were up to 100A of loads in the subpanel, you
would not exceed the allowance for the feeder or the sub-panel. Still,
if you do have loads in the subpanel, you have to up sizes the
sub-panel, but not the feeder conductor.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
www.berkeleysolar.com <http://www.berkeleysolar.com/>   

________________________________

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Wednesday, March 30, 2011 5:18 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used
tocombinemultipleinverter outputs

Another key is to remember that this discussion also applies to the
conductor between the main panel and subpanel. In a large PV system,
this could result in a pretty large wire between the two panels, and a
significant cost that is often overlooked. In some cases it makes sense
to locate the subpanel close to the main panel and run multiple sets of
smaller wires from the inverters to the subpanel.

And because the calculation is based on the first OC protection
connected to the inverters, adding a main breaker (theoretically 80A in
this example) in the subpanel doesn't change things. Even though this
wire would be theoretically protected by an 80A breaker at each end, you
can't size the wire for 160A / 1.2 = 133.3A. You have to size for
180A/1.2 = 150A. (not that it makes much of a difference in this
example, but it still must be considered)

At least that's how I understand it...

Jason Szumlanski

Fafco Solar 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark
Frye
Sent: Wednesday, March 30, 2011 1:33 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to
combinemultipleinverter outputs

Opps!

My bad, I was thinking of a single phase system, not the three phase
system shown in the article.

For the three phase system Kent is correct in counting 180A of supply
per bar.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
www.berkeleysolar.com <http://www.berkeleysolar.com/>   

________________________________

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark
Frye
Sent: Tuesday, March 29, 2011 10:17 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine
multipleinverter outputs

I think Kent and I agree. For the case where the subpanel is not
dedicated a PV sub-panel he is calculating for 2 - 50A breakers and I
calculated for 3 - 50A breakers.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
www.berkeleysolar.com <http://www.berkeleysolar.com/>   

________________________________

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
Osterberg
Sent: Tuesday, March 29, 2011 9:26 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple
inverter outputs

Per 705.12(D) the sub-panel could be any distribution equipment on the
premises. So the question becomes: is the sub-panel capable of supplying
branch circuits or feeder loads? If yes, then the sum of the breakers
(potentially) feeding the bus is 180 amps so a 150-amp rating is
required and the inverters would have to feed the opposite end of the
bus bars. If no, the code is not clear on the requirement, but obviously
the 80-amp breaker in the main panel limits the maximum current flowing
through the sub-panel. 

Kent Osterberg
Blue Mountain Solar

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